1. Field of the Invention
This invention is related to the field of integrated circuits and, more/particularly, to input/output driver circuits.
2. Description of the Related Art
Integrated circuits generally include core circuitry that implements the operation for which the integrated circuit is designed, driver circuitry to drive output signals from the integrated circuit to external circuitry, and receiver circuits to receive input signals from external circuitry. The driver/receiver circuitry buffers and isolates the core circuitry from the external circuitry, handling the larger loads, higher current flows, higher voltages, noise, etc. involved in external communication.
Originally, the core circuitry operated with the same power supply voltage as the driver/receiver circuitry. However, as semiconductor fabrication technology continued to evolve and transistor feature sizes continued to be reduced, the core circuitry eventually required power supply voltages lower than those that could be used for communicating with the external circuitry. In some cases, backward compatibility with legacy external circuitry that was not manufactured using the most advanced semiconductor fabrication technology was desired. In other cases, a higher communication voltage is required by the effects of noise and other factors that affect the reliability of external communications.
The driver/receiver circuitry designs have changed to handle the differences in internal supply voltages and external communication voltages. For example, transistors used in the driver/receiver circuitry can implement feature sizes that are larger than the transistors used in the core circuitry, to safely handle the higher voltages. Level shifting techniques can be used to translate signals from the core circuitry domain to the driver/receiver domain, and vice versa.
For long term reliability reasons, the voltage drop across any two terminals of the transistors needs to be limited to a specified maximum. If the voltage drop consistently exceeds the maximum, the transistors will cease to function (or “burn out”). The oxide between the gate and the semiconductor substrate can be destroyed, for example. The channel can be destroyed as well, if the drain to source voltage exceeds the maximum.